Recently, many recording systems have been studied and developed and some of them have been practically used. Among them, heat or pressure sensitive recording materials are widely used for an output of a computer, a printer of an electric calculator, a recorder for medical inspection, a facsimile, an automatic ticket machine, copying and the like, because they do not need complicated treatments for developing, fixing and the like, and can record in a short period of time by a simple and less expensive apparatus without making substantial noise.
The heat or pressure sensitive recording materials work by bringing an electron donating colorless dye into chemical contact with an electron accepting compound to cause color change of the colorless dye. The chemical contact can be carried out by heating using a thermal head, a heat pen, laser beam and the like, or by pressing using a pencil and the like to give a colored image.
As the electron accepting compound (color developer), a compound having a phenolic OH group is generally proposed (Japanese Patent Publications (examined) 9309/1965, 4160/1968, 14039/1970 and 29830/1976, and Japanese Kokai Publication (unexamined) 144193/1981, and the like). Practically employed are bisphenol compounds and 4-hydroxybenzoic acid esters. The color developers require the following properties: (1) to provide images having a high color strength; (2) to provide a stable developed image having no color change by means of moisture, light, oil, time and the like; (3) less water solubility; (4) no subliming properties; (5) producible on an industrial scale and obtainable at a high yield and purity at a low cost; and the like. The above proposed color developers do not meet all requirements For example, some of them do not sharply respond to heat and therefore do not develop desired color strength in case of high speed recording. Others have defects of changing color with time and forming white spots on the developed color image.
In order to overcome the above defects, methods using a sensitizer or a stabilizer are proposed in Japanese Kokai Publications (unexamined) 34842/1974, 106746/1977 and 39139/1978. These methods do not attain desired improvement levels and give rise to other defects of discoloration by oil or other reasons.